Why Are There Gay Men?

While female sexuality appears to be more fluid, research suggests that male gayness is an inborn, unalterable, strongly genetically influenced trait. But considering that the trait discourages the type of sex that leads to procreation — that is, sex with women — and would therefore seem to thwart its own chances of being genetically passed on to the next generation, why are there gay men at all?

Put differently, why haven't gay man genes driven themselves extinct?

This longstanding question is finally being answered by new and ongoing research. For several years, studies led by Andrea Camperio Ciani at the University of Padova in Italy and others have found that mothers and maternal aunts of gay men tend to have significantly more offspring than the maternal relatives of straight men. The results show strong support for the "balancing selection hypothesis," which is fast becoming the accepted theory of the genetic basis of male homosexuality.

The theory holds that the same genetic factors that induce gayness in males also promote fecundity (high reproductive success) in those males' female maternal relatives. Through this trade-off, the maternal relatives' "gay man genes," though they aren't expressed as such, tend to get passed to future generations in spite of their tendency to make their male inheritors gay.

While no one knows which genes, exactly, these might be, at least one of them appears to be located on the X chromosome, according to genetic modeling by Camperio Ciani and his colleagues. Males inherit only one X chromosome — the one from their mother — and if it includes the gene that promotes gayness in males and fecundity in females, he is likely to be gay while his mom and her female relatives are likely to have lots of kids. If a daughter inherits that same X-linked gene, she herself may not be gay, but she can pass it on to her sons. [Why Are There Gay Women?]

But how might the "gay man gene" make females more reproductively successful? A new study by Camperio Ciani and his team addresses the question for the first time. Previously, the Italian researchers suggested that the "gay man gene" might simply increase androphilia, or attraction to men, thereby making the males who possess the gene homosexual and the females who possess it more promiscuous. But after investigating the characteristics of 161 female maternal relatives of homosexual and heterosexual men, the researchers have adjusted their hypothesis. Rather than making women more attracted to men, the "gay man gene" appears to make these women more attractive to men.

"High fecundity, that means having more babies, is not about pleasure in sex, nor is it about promiscuity. The androphilic pattern that we found is about females who increase their reproductive value to attract the best males," Camperio Ciani told Life's Little Mysteries.

Turns out, the moms and aunts of gay men have an advantage over the moms and aunts of straight men for several reasons: They are more fertile, displaying fewer gynecological disorders or complications during pregnancy; they are more extroverted, as well as funnier, happier and more relaxed; and they have fewer family problems and social anxieties. "In other words, compared to the others, [they are] perfect for a male," Camperio Ciani said. Attracting and choosing from the best males enables these women to produce more offspring, he noted.

The new study will appear in an upcoming issue of the Journal of Sexual Medicine.

Of course, no single factor can account for the varied array of sexual orientations that exist, in men as well as in women. "It is quite possible that there are several influences on forming a homosexual orientation," said Gerulf Rieger, a sexual orientation researcher at Cornell University. He noted that environmental factors — including the level of exposure to certain hormones in the womb — also play a role in molding male sexuality. But as for why genetic factors would exist that make men gay, it appears that these genes make women, as well as gay men, alluring to other men.

Natalie Wolchover was a staff writer for Live Science from 2010 to 2012. She hold a bachelor's degree in physics from Tufts University and has studied physics at the University of California, Berkeley. Follow Natalie on Google+.